5 ms·

exo762's reply is making my head hurt so here's a link in case you're curious as to how the tides are making the moon spiral outwards. http://www.windows2unive

exo762's reply is making my head hurt so here's a link in case you're curious as to how the tides are making the moon spiral outwards. http://www.windows2universe.org/kids_space/moon_orbit.html http://www.windows2universe.org/kids_space/moon_orbit.html TL; DR - The earth's rotation drags the tidal bulge faster than the moon's orbit slowing the earth and giving more energy to the moon which moves it into a higher orbit.

stephth 13y ago
Thanks for the link, I appreciated the short but informative explanation. Today I learned the moon is spinning away from the Earth while the Earth's rotation is slowing down, and by the time this dance movement ends, a day on the Earth will last a month!
Tuna-Fish 13y ago
... I've known about the widening orbit of the moon, but I only just realized that if the moon was rotating in a different direction, it would eventually spiral down to the earth. Imagine living on such a planet, with a large gravity capture moon, that provides a definite sell by date to living on the surface.
- mixmastamyk 13y ago
  Was thinking about that too. It is sad that the moon is leaving us, but the opposite would be a lot worse. Could we survive such an event? Perhaps by chilling on Mars for a few thousand? years.
  - Tuna-Fish 13y ago
    Few thousand would be lowballing it rather lot. An impact of that magnitude would melt the entire surface, boil the oceans, and strip off the atmosphere. It would take hundreds of millions of years for earth to recover, if it ever would. Also, Mars is not that hospitable to life. The best option I see would be putting as much people and industry in orbit as possible, and try to live from asteroids.
    - ars 13y ago
      > An impact of that magnitude It wouldn't be an impact. For the moon to reduce its orbit to the point that it impacted the earth it would be moving rather slow as orbital objects go. It would be more of a gradual merging as chunks of rock and ocean moved between the two bodies.
      - rohansingh 13y ago
        Exactly. See: http://en.wikipedia.org/wiki/Roche_limit http://en.wikipedia.org/wiki/Roche_limit Basically, in this scenario the moon would either come down in chunks or would form a set of rings.
        mixmastamyk 13y ago
        Yes, I was thinking a -3.8cm per year rate, and did contemplate the "squishing" that the moon Io has to deal with. Just pulled the timeframe from my rear end, probably would take that long just to get the process started. :/
    - bronson 13y ago
      As inhospitable as Mars is, asteroids and space stations are far worse!
      - mixmastamyk 13y ago
        Yep, and faced with an end-of-life event we'd probably make a go of it, no?
    - anonymous 13y ago
      > Mars is not that hospitable to life I've always found it rather interesting how the combined mass of Mars and Venus is just about the same as Earth's. Can't we move Venus out to a higher orbit where it would impact with Mars and produce an Earth 2?
      - 13y ago
  - deleted 13y ago
    [deleted]
- Nursie 13y ago
  We already have a sell-by date. Long before the sun goes red giant, in fact in about a billion years from now, solar output will increase to the point that liquid water (and therefore life) will be impossible on the surface of the earth.
- deleted 13y ago
  [deleted]
- ars 13y ago
  In such a world all the extra energy would have to be captured by tides. Right now a portion of the energy is used to "fling" the moon farther away, and a portion is used by tides. In the other scenario not only is all the energy available to tides, but you also have the extra added energy of the moon "falling" toward the earth. I'm not sure tides could capture such energy without smoothing the borders of the oceans, which would cause less friction, and then consequently less momentum transfer to the moon.
- MichaelApproved 13y ago
  Wouldn't the moon leaving the Earth's orbit give us a sell-by date too? I thought the removal of a large object like the moon would change our orbit as well, causing things to be dramatically different on Earth. Is that wrong? What would happen to the Earth when the moon breaks away?
  - andrewflnr 13y ago
    That's a pretty big if, actually. It may be possible for the earth to give up all its angular momentum without giving the moon enough energy to escape. But then we have eternal day and night on opposite sides of earth, so that's probably not a big improvement.
    - adaml_623 13y ago
      Actually the angular momentum transfer will stop when the moons orbit is the same length as the earth's rotation. So earth would still have day and night but would only have 10-12 per year.
ChuckMcM 13y ago
And of course for the curious, no the moon was not 'skimming the surface of the earth' a million years ago as would be implied by a 'continual' increase of 3.8 cm/yr in the orbit. The earth is not homogenous, and so the gravitational force experienced by the moon is not constant. This results in a transfer of angular momentum between the two, speeding up the moon slightly (and slowing the earth even more slightly as the transfer is proportional to mass)
- jacquesm 13y ago
  Even over a billion years that would only change the distance by 38,000 km which means it wasn't 'skimming the surface of the earth' by any imagination. Unless there is a 260,000 Km inclusive definition of skimming. Even at 4.6 billion years (assuming a linearity for that 3.8 cm / year that likely isn't there) that would still only account for just over half the earth-moon distance. So I don't think the skimming was implied in any way!
  - ChuckMcM 13y ago
    You are correct of course. Now I feel sad that I didn't do the math correctly in my head (in my world apparently kilometers are like 100M or something :-()
    - jacquesm 13y ago
      I wouldn't worry about it, the mental image of the moon skimming the surface of the earth was worth it :) But at the required orbital speeds the tidal effects would be another matter. Talk about a rip-tide :)
educating 13y ago
> TL; DR - The earth's rotation drags the tidal bulge faster than the moon's orbit slowing the earth and giving more energy to the moon which moves it into a higher orbit. Is there no way that this is a misinterpretation of the facts? Couldn't the answer be that the Earth is losing mass through its atmosphere (into space), and due to having less mass, the moon has less gravitational pull with the Earth, and the orbit becomes larger?
- adaml_623 13y ago
  A good question. However if the Earth was becoming lighter then that would affect the gravitational interaction with lots of things like satellites, passing asteroids and maybe our orbit around the Sun. I'm quite sure the change with all the satellites would be easily noticed.

stephth 13y ago

Thanks for the link, I appreciated the short but informative explanation. Today I learned the moon is spinning away from the Earth while the Earth's rotation is slowing down, and by the time this dance movement ends, a day on the Earth will last a month!

Tuna-Fish 13y ago

... I've known about the widening orbit of the moon, but I only just realized that if the moon was rotating in a different direction, it would eventually spiral down to the earth. Imagine living on such a planet, with a large gravity capture moon, that provides a definite sell by date to living on the surface.

mixmastamyk 13y ago
Was thinking about that too. It is sad that the moon is leaving us, but the opposite would be a lot worse. Could we survive such an event? Perhaps by chilling on Mars for a few thousand? years.
- Tuna-Fish 13y ago
  Few thousand would be lowballing it rather lot. An impact of that magnitude would melt the entire surface, boil the oceans, and strip off the atmosphere. It would take hundreds of millions of years for earth to recover, if it ever would. Also, Mars is not that hospitable to life. The best option I see would be putting as much people and industry in orbit as possible, and try to live from asteroids.
  - ars 13y ago
    > An impact of that magnitude It wouldn't be an impact. For the moon to reduce its orbit to the point that it impacted the earth it would be moving rather slow as orbital objects go. It would be more of a gradual merging as chunks of rock and ocean moved between the two bodies.
    - rohansingh 13y ago
      Exactly. See: http://en.wikipedia.org/wiki/Roche_limit http://en.wikipedia.org/wiki/Roche_limit Basically, in this scenario the moon would either come down in chunks or would form a set of rings.
      - mixmastamyk 13y ago
        Yes, I was thinking a -3.8cm per year rate, and did contemplate the "squishing" that the moon Io has to deal with. Just pulled the timeframe from my rear end, probably would take that long just to get the process started. :/
  - bronson 13y ago
    As inhospitable as Mars is, asteroids and space stations are far worse!
    - mixmastamyk 13y ago
      Yep, and faced with an end-of-life event we'd probably make a go of it, no?
  - anonymous 13y ago
    > Mars is not that hospitable to life I've always found it rather interesting how the combined mass of Mars and Venus is just about the same as Earth's. Can't we move Venus out to a higher orbit where it would impact with Mars and produce an Earth 2?
    - 13y ago
- deleted 13y ago
  [deleted]
Nursie 13y ago
We already have a sell-by date. Long before the sun goes red giant, in fact in about a billion years from now, solar output will increase to the point that liquid water (and therefore life) will be impossible on the surface of the earth.
deleted 13y ago
[deleted]
ars 13y ago
In such a world all the extra energy would have to be captured by tides. Right now a portion of the energy is used to "fling" the moon farther away, and a portion is used by tides. In the other scenario not only is all the energy available to tides, but you also have the extra added energy of the moon "falling" toward the earth. I'm not sure tides could capture such energy without smoothing the borders of the oceans, which would cause less friction, and then consequently less momentum transfer to the moon.
MichaelApproved 13y ago
Wouldn't the moon leaving the Earth's orbit give us a sell-by date too? I thought the removal of a large object like the moon would change our orbit as well, causing things to be dramatically different on Earth. Is that wrong? What would happen to the Earth when the moon breaks away?
- andrewflnr 13y ago
  That's a pretty big if, actually. It may be possible for the earth to give up all its angular momentum without giving the moon enough energy to escape. But then we have eternal day and night on opposite sides of earth, so that's probably not a big improvement.
  - adaml_623 13y ago
    Actually the angular momentum transfer will stop when the moons orbit is the same length as the earth's rotation. So earth would still have day and night but would only have 10-12 per year.

ChuckMcM 13y ago

And of course for the curious, no the moon was not 'skimming the surface of the earth' a million years ago as would be implied by a 'continual' increase of 3.8 cm/yr in the orbit. The earth is not homogenous, and so the gravitational force experienced by the moon is not constant. This results in a transfer of angular momentum between the two, speeding up the moon slightly (and slowing the earth even more slightly as the transfer is proportional to mass)

jacquesm 13y ago
Even over a billion years that would only change the distance by 38,000 km which means it wasn't 'skimming the surface of the earth' by any imagination. Unless there is a 260,000 Km inclusive definition of skimming. Even at 4.6 billion years (assuming a linearity for that 3.8 cm / year that likely isn't there) that would still only account for just over half the earth-moon distance. So I don't think the skimming was implied in any way!
- ChuckMcM 13y ago
  You are correct of course. Now I feel sad that I didn't do the math correctly in my head (in my world apparently kilometers are like 100M or something :-()
  - jacquesm 13y ago
    I wouldn't worry about it, the mental image of the moon skimming the surface of the earth was worth it :) But at the required orbital speeds the tidal effects would be another matter. Talk about a rip-tide :)

educating 13y ago

> TL; DR - The earth's rotation drags the tidal bulge faster than the moon's orbit slowing the earth and giving more energy to the moon which moves it into a higher orbit. Is there no way that this is a misinterpretation of the facts? Couldn't the answer be that the Earth is losing mass through its atmosphere (into space), and due to having less mass, the moon has less gravitational pull with the Earth, and the orbit becomes larger?

adaml_623 13y ago
A good question. However if the Earth was becoming lighter then that would affect the gravitational interaction with lots of things like satellites, passing asteroids and maybe our orbit around the Sun. I'm quite sure the change with all the satellites would be easily noticed.